The human heart normally maintains its own well-ordered intrinsic rhythm through generation of stimuli by pacemaker tissue that results in a wave of depolarization that spreads through specialized conducting tissue and then into and through the myocardium. The well-ordered propagation of electrical depolarizations through the heart causes coordinated contractions of the myocardium that results in the efficient pumping of blood. In a normally functioning heart, stimuli are generated under the influence of various physiological regulatory mechanisms to cause the heart to beat at a rate that maintains cardiac output at a level sufficient to meet the metabolic needs of the body. Abnormalities of excitable cardiac tissue, however, can lead to abnormalities of heart rhythm that are called arrhythmias. All arrhythmias stem from one of two causes: abnormalities of impulse generation or abnormalities of impulse propagation. Arrhythmias can cause the heart to beat too slowly (bradycardia, or a bradyarrhythmia) or too quickly (tachycardia, or a tachyarrhythmia), either of which may cause hemodynamic compromise or death.
Drug therapy is often effective in preventing the development of arrhythmias and in restoring normal heart rhythms once an arrhythmia has occurred. However, drug therapy is not always effective for treating particular arrhythmias, and drug therapy usually causes side-effects that may be intolerable in certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of an implantable cardiac rhythm management device that delivers therapy to the heart in the form of electrical stimuli. Such devices include cardiac pacemakers that deliver timed sequences of low energy electrical stimuli, called pacing pulses, to the heart via an intravascular lead having one or more electrodes that are disposed in the myocardium of the paced chamber. Heart contractions are initiated in response to such pacing pulses, and by properly timing the delivery of the pacing pulses, the heart can be made to contract in proper rhythm, greatly improving its efficiency as a pump. Such pacemakers are often used to treat patients with bradycardia due either to conduction abnormalities (e.g., AV block) or to sinus node dysfunction.
Cardiac rhythm management devices may also be used in the treatment of tachyarrhythmias such as tachycardia (i.e., a heart rate that is too rapid). Pacemakers, for example, can be configured to deliver paces to the heart in such a manner that the heart rate is slowed, a pacing mode referred to as anti-tachycardia pacing. Another class of cardiac rhythm management devices, implantable cardioverter/defibrillators (ICD's), deliver high energy electrical stimuli to the heart in order to terminate fibrillation, which is the most extreme form of tachyarrhythmia. Fibrillation, which may occur in either the atria or the ventricles, refers to the situation where electrical activity spreads through the myocardium in a disorganized fashion so that effective contraction does not occur. An ICD delivers a high energy electrical stimulus or shock to either the atria or ventricles in order to terminate the fibrillation, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to ICD's and pacemakers, cardiac rhythm management systems also include pacemaker/ICD's that combine the functions of pacemakers and ICD's, drug delivery devices, and any other implantable or external systems or devices for diagnosing, monitoring, or treating cardiac arrhythmias.
Irregular ventricular tachycardia, in which the ventricles beat more rapidly and irregularly than normal, can be due to a variety of etiologies. Certain patients, for example, are prone to premature ventricular contractions due to ectopic excitatory foci in the ventricular myocardium. Another cause of ventricular tachycardia is atrial fibrillation. The intrinsic ventricular rhythm that occurs during an episode of atrial fibrillation is a result of the chaotically occurring depolarizations occurring in the atria being passed through the AV node to the ventricles. The intrinsic ventricular rate is then governed by the cycle length of the atrial fibrillation and the refractory period of the AV node. Although the intrinsic ventricular rate is less than the atrial rate, due to the refractory period of the AV node, it is still rapid and irregular. When the ventricles contract at irregular intervals, the contraction can occur prematurely before diastolic filling is complete which decreases the stroke volume for that contraction. This can be especially significant in, for example, congestive heart failure patients who are already hemodynamically compromised. Concomitant atrial fibrillation where the atria no longer act as effective priming pumps can also contribute to the problem. An irregular ventricular rate can thus depress cardiac output and cause such symptoms as dyspnea, fatigue, vertigo, and angina.
An objective of the present invention is to use pacing therapy to maintain hemodynamic stability in the presence of an irregular intrinsic ventricular rhythm. Such pacing therapy may be used in conjunction with any type of cardiac rhythm management device that is capable of delivering ventricular paces.